In microelectronic applications, polymers that demonstrate high temperature resistance are generally well known. Precursors of such polymers, such as polyimides and polybenzoxazoles can be made photoreactive with suitable additives. The precursors are converted to the desired polymer by known techniques such as exposure to high temperatures. The polymer precursors are used to prepare protective layers, insulating layers, and relief structures of highly heat-resistant polymers.
Conventional positive-working photosensitive polybenzoxazoles (PBO) contain an alkaline soluble PBO precursor and a diazoquinone photoactive compound as shown in U.S. Pat. No. 4,371,685. The diazoquinone compound inhibits the solubility of the PBO precursor in an aqueous base. After exposure to light, the diazoquinone compound undergoes photolysis and converts to indenecarboxylic acid, which promotes the aqueous base solubility of the PBO precursor.
The PBO precursors disclosed in U.S. Pat. No. 4,371,685 have amino end groups. Amino groups are known to cause decomposition of diazoquinone moieties (see for example Arnost Reiser, Photoreactive Polymer, John Wiley and Sons, Inc, 1989) which limits the shelf life of the photosensitive formulations based on PBO precursors and diazoquinone compounds.
In order to remedy this problem it was proposed in U.S. Pat. No. 6,376,151 B1 to convert amino end groups of PBO precursor into amide groups mostly through the reaction with various anhydrides or sulfonyl chlorides. In many cases it was demonstrated that the shelf life of formulations based on such modified PBO precursors is indeed substantially better than the shelf life of formulations based on PBO precursors containing amino end groups.
It was shown furthermore that the attachment of some diazoquinone moieties directly to the PBO precursor backbone, as disclosed in U.S. Pat. No. 6,177,225 B1 and U.S. Pat. No. 6,127,086, dramatically improves such lithographic properties of the positive-working photosensitive formulations as contrast and photospeed. However, these PBO precursors still have amino end groups, which also put limitations on the shelf life of their photosensitive formulations.
Cured PBO formulations are typically dark in color because of the myriad of high temperature reactions which take place during the high temperature cure. Unfortunately, the color of cured films of the positive photosensitive formulations is important for alignment during wire bonding and other packaging and assembly operations. Lighter-colored, more transparent films offer significant advantages in alignment during wire bonding and other packaging and assembly operations. This can result in increased throughput and accuracy.
The present invention discloses new photosensitive PBO precursor, which contains diazoquinone moieties attached to its backbone and in which all amino end groups are converted into amide groups. This invention found that the applicable end groups could be of much wider variety than those identified in U.S. Pat. No. 6,376,151 B1. The photosensitive formulation based on the disclosed PBO precursor has good imaging and mechanical properties as well as superior shelf life stability.
Furthermore, we have surprisingly found that photosensitive polybenzoxazole precursor polymers having (sulfon)amide end groups (with or without attached diazoquinone groups) can be formulated into photosensitive compositions with diazoquinone photoactive compounds which lack benzylic hydrogens on the backbone to yield compositions producing films significantly lighter in color after curing. Previous patents disclosing positive polybenzoxazole photosensitive formulations using diazoquinone photoactive compounds which lack benzylic hydrogens on the PAC backbone did not disclose any report about the color of the resulting cured films of such formulations nor how to address the problem of dark cured film in PBO formulations. Such formulations are disclosed in U.S. Pat. Nos. 5,376,499, U.S. Pat. No. 5,726,279, U.S. Pat. No. 5,783,654, U.S. Pat. No. 5,883,221, U.S. Pat. No. 5,922,825, U.S. Pat. No. 5,973,202, U.S. Pat. No. 6,120,970 or U.S. Pat. No. 6,153,350, herein incorporated by reference.